Traditionally, europium activated barium thioaluminate or barium magnesium thioaluminate thin film phosphors have been used in thick film dielectric electroluminescent displays. While the performance of these phosphor materials has been substantially enhanced with the use of advanced deposition and crystallization methods as well as through improvements to the overall display structure, particularly to the provision of chemical barrier layers and electron injection enhancement layers adjacent to the phosphor film, it is still desirable to have a phosphor material with a higher luminance and energy efficiency than that afforded by these phosphor materials as they are known in the art.
The BaAl2S4 and BaAl4S7 compounds of the prior art have each have been identified to have one crystal structure. For BaAl2S4, referred to herein as BaAl2S4 (I), this is a cubic lattice with a lattice constant a of 12.65 Angstrom units and containing 12 BaAl2S4 molecules in the crystal unit cell arranged according to the crystal symmetry-defining Hermann Mauguin space group Pa-3. The definition of the Hermann Mauguin space groups can be found in the Lawrence Livermore National Laboratory (LLNL) website http://www.structure.llnl.gov/xray/comp/space instr.htm. The crystal structure of this compound as described above and determined from x-ray diffraction data is published in Materials Research Bulletin Volume 17 (1992), page 1169. This crystal structure for BaAl2S4(I) is characterized in having an x-ray diffraction reflection at a diffraction angle of θ=at 15.7 degrees and at 23.3 degrees when Cu Kα x-rays are used to generate the diffraction data. This crystal structure has 12 atomic sites containing a barium atom, 8 of which are equivalent by symmetry elements of the space group and the remaining 4 of which are also related by symmetry elements of the space group, but the two groups of barium atoms occupy atomically inequivalent sites in the crystal lattice. It is understood that if the compound is doped with europium, which is chemically similar to barium, the europium atoms will occupy atomic sites normally occupied by barium atoms, and so there are two distinct types of sites that the europium atoms may occupy, thus giving rise to different electroluminescent emission characteristics from the europium corresponding to each type of site.
The previously known crystal structure for BaAl4S7, herein referred to as BaAl4S7 (I), has an orthorhombic lattice with lattice constants a=14.81 Angstroms, b=6.22 Angstroms and c=5.89 Angstroms and contains 2 BaAl4S7 molecules in the crystal unit cell arranged according to the crystal symmetry-defining Hermann Mauguin space group P m n 21. This crystal structure has two atomic sites containing barium in the unit cell that are related to each other by the symmetry elements of the space group, so there is only one type of site into which europium can be substituted. The crystal structure of this compound has been determined from x-ray diffraction data as described in Eisenmann et al in Rev. Chim Miner. Volume 20 (1983), pg. 329. It density as calculated from the crystallographic data is 2.88 grams per cubic centimeter.
The performance of phosphor materials may be enhanced if the elemental composition of a deposited film is commensurate with the stoichiometry of the desired phosphor compound, however, the prior art does not anticipate the existence of more than one crystal phase for a given stoichiometry, nor does it teach or suggest the performance advantage that can be realized by ensuring that the phosphor material is formed from a preferred crystal phase or a mixture of preferred crystal phases.
It is therefore desirable to provide barium thioaluminate phosphor compounds with increased electroluminescent light emission compared to those of the prior art.